how much volume can polyfil account for?

I am getting frusterated as to how i am going to fit my sub/subs in my car and was wondering how much volume polyfill can add at a maximim and approximately what is the ratio of open space vs polyfil inside a box?

also, i have heard that polyfil makes it significantly harder to design and tune a ported box... are there any tips that i am unaware of?

i am going to try to get one or two either 15" L7's or 15" CVX's. i can get about three cubic feet in my trunk without too much dificulty, but any more and i cant get to my spair tire (which is now becoming vital).

"big slicks, you know its me. burnin cigs, you know its me. pistol on the seat, extra clip, yea, you know its me."